In one stunning instant, the “pop” of gunfire began exploding, and Steven Rayle froze in horror as bodies began to slump to the ground.

“There was a sense of disbelief,” the 52-year-old Tucson doctor told The Post. He was standing 10 feet away from yesterday’s sickening ambush on Rep. Gabrielle Giffords and 20 people who had gathered for her “Congress on Your Corner” event at a Safeway supermarket.

“There was surreal feeling to it. Gunshots in real life don’t sound real.”

Rayle told The Post in a telephone interview that the gathering seemed strictly “low-key.”

“It was the most ordinary of mornings,” he said. “No protesters. And then this happened.

“I saw Rep. Giffords behind the table right in front of the store talking to a couple,” he said. “I went beyond the table to the side, . . . looked up, and in a second or two a man came up and pulled a gun and shot her in the head.

“He began shooting people just randomly, just rapid-fire at whomever was around there,” the doctor continued. “I ducked behind a concrete post. He began coming beyond that.”

To avoid the gunman’s wrath, the doctor lay down and played dead.

“It was totally surreal,” Rayle said. “He was just shooting at everyone — open fire at everyone.

“[Someone] tackled him and jumped on him. I lay there for a few seconds. He ran out of ammunition.”

In the wake of the fusillade, there was a macabre calm, Rayle said.

“There was not a sense of pandemonium,” he said. “No one was hysterical or screaming. We just began — people calling for help.”

Rayle said the suspect, whom he helped hold for cops, “didn’t say anything.”

In the aftermath, Rayle, an ER physician, said he tried to help the wounded. “I did CPR and checked on other people as well — coordinated triage,” he said.

“She was propped up, [with a] wound on the left side of her forehead,” he said. “She was moving her extremities. I felt somewhat hopeful she would be all right.”

Matthew Laos, 43, was the first person in line at Giffords’ event, hoping to talk to the congresswoman about his Army assignment and to show her an award he’d received, according to the Arizona Daily Star.

“I was proud to show her the award. And I even said to her that I was so proud she had won this election under the most difficult circumstances,” he said.

As he drove away, he said he saw cop cars whizzing by — and given how close it was to the event he’d just left, he said he got a strange feeling. Turning on the TV, he learned the horrifying news, and then raced to Giffords’ office to get more information.

More than a dozen others gathered there, some bringing flowers.

Jason Pekau, an employee at a Sprint store near the supermarket, said the sound of “15 to 20 gunshots” sent him racing into the parking lot that fronts his workplace and the Safeway where Giffords had set up her event.

“I just saw people running, screaming, from the shooting,” he recalled. “Everyone screamed it was Gabrielle Giffords. I saw her on the stretcher . . . She was moving, from what I saw.”